How many people does it affect worldwide?

Because of the many forms modern slavery takes, it is impossible to estimate accurately the numbers involved.

To take just one example, the International Labour Organisation (ILO) estimates that around 200 million children around the world are used as child labour.

Many millions are roped into bonded labour, in which poor people are forced or tricked into taking a loan, for example to pay for trafficking to a developed country or medical treatment and then made to work indefinitely.

The ILO estimates at least 12.3 million people are trapped in various forms of forced labour, including debt bondage, forced prostitution and sweatshop workers illegally paid minimal sums.

Where does it take place?

Slavery occurs throughout the world, even in developed countries such as the UK and the US.

Thousands of trafficked women are forced into the sex industry and others face many years of servitude to pay back loans to people smugglers who brought them into the country.

Some countries, particularly in west Africa, have traditions of slavery related to caste or class divisions that remain extremely resistant to laws banning them.

In Mauritania, where slavery was officially outlawed in 1981, some human rights groups estimate that up to 20% of the country's three million population still live as slaves.

Interviews with more than 11,000 people led to an estimate that at least 43,000 people across the country are enslaved.

The study found these slaves are owned and controlled by their masters. All they receive is minimal food and a place to sleep. The master decides who a slave marries and whether their children go to school.

The US state department produces a regular report on human trafficking, which it calls a new type of global slave trade. It ranks countries in tiers according to how serious their problems with human trafficking are deemed to be.

The 16 lowest-ranked, or tier three nations, in the 2007 report include seven Middle East nations (Bahrain, Iran, Oman, Syria, Kuwait, Qatar, Saudi Arabia), three from Africa (Equatorial Guinea, Sudan, Algeria) and three from Asia (North Korea, Burma, Malaysia), as well as Uzbekistan, Cuba and Venezuela.